1. Field of the Invention
The present invention relates to a solid-state imaging device in which photoelectric converters are arranged in a two-dimensional matrix, and to a technique of providing a function for significantly increasing the number of images taken in high-speed photography and enabling multiple exposure photography.
2. Related Art
High-speed cameras are used in detail analyses such as a car crash test and a product drop test. Especially in recent years, high-speed cameras are also used in analyses of phenomena in physical chemistry such as a combustion state in an internal combustion engine and a chemical reaction.
Also, with the advancement of digital image processing systems, the use of high-speed digital cameras that adopt solid-state imaging devices such as a CCD (Charge Coupled Device) and a MOS image sensor becomes increasingly predominant. In such systems, there is no need to develop a film after photographing unlike conventional film cameras, and a photographed image can be checked instantly. In addition, the image can be easily provided into a digital image analyzing device using a personal computer and the like.
Since the photographed image can be checked instantly, even when a photographing error occurs, it can be easily dealt with by, for example, reshooting on the spot. For processes such as data reuse and data processing, too, high-speed digital cameras that lend themselves to use for image processing and image alteration on personal computers are suitable.
However, in such high-speed cameras, the number of frames that can be photographed successively is limited due to a factor such as a speed of writing onto a recording medium. To continuously photograph a moving image, it is necessary to confine a frame rate and an image size to such ranges that can be kept up with by the recording medium writing speed. When performing high-speed photography with a high resolution, since the number of photographable frames is limited and also a shooting interval is short, a photographing period will end up being very short. For example, when the frame rate is 10,000 frames per second and the number of photographable frames is 100, the photographing period is only 10 milliseconds.
Even when a solid-state imaging device that has storage cells in a pixel is employed, reading data takes a long time, because the number of storage cells is limited and also the same number of reading operations as the storage cells need to be performed to read data from these storage cells. Accordingly, in the case where high-speed photography is performed the same number of times as the storage cells and, after reading data from the storage cells, high-speed photography is again performed the same number of times as the storage cells, a photographing operation is stopped during when the data is being read, so that only information which is discrete in time can be obtained.
A device that achieves high-speed photography by storing information output from a solid-state imaging device into storage cells is conventionally available as disclosed in Japanese patent application publication No. 2000-165750. However, the number of storage cells is limited to about several tens to several hundreds, and it is impossible to store a larger number of pieces of information than the number of storage cells. Also, reading data requires a time period corresponding to the number of storage cells.
For multiple exposure photography that combines images of a fast moving object at discrete times into a single image, a conventional film camera requires an external device such as a strobe light capable of emitting pulses of light successively at high speed. Though the use of an electronic shutter of a CCD or the like enables discrete images to be obtained, high-speed photography is difficult. Even when a solid-state imaging device is used, an external device such as a high-speed mechanical shutter and a high-speed multi strobe light is necessary. Thus, high-speed multiple exposure photography by a camera alone has not been realized yet.